test: follow-up to rpc: default maxfeerate value as BCH/kB

Summary:

• test: fix incorrect value in rpc_rawtransaction.py
• doc: improve rawtransaction code/test docs
• test: use named args for sendrawtransaction calls

involving more than one argument.

This is a backport of Core PR16929

Depends on D8027

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8035